1. Field of the Invention
The present invention relates to an insulating thin film, a formation solution for the insulating thin film, a method for manufacturing the insulating thin film, a field-effect transistor, a method for manufacturing the field-effect transistor and an image display unit. Particularly, the present invention relates to the insulating thin film having high insulating properties and high electric permittivity, the formation solution for the insulating thin film, the method for manufacturing the insulating thin film, the field-effect transistor, the method for manufacturing the field-effect transistor and the image display unit.
2. Description of the Related Art
In recent years, most of the display units which are widely used in general such as a liquid crystal display, an organic light emitting diode (OLED) display and an electrophoretic form display employ a drive unit of an active matrix type in which a thin film transistor (TFT) is utilized as a display switching device. The field-effect transistor (FET) having a gate electrode, a gate insulating layer, a source electrode-a drain electrode and a semiconductor arranged between the source electrode and the drain electrode is utilized as the TFT for this kind of the display switching device. A drive principle of FET is to control an amount of a charge carrier of an electron or a hole of the semiconductor by applying a voltage to the gate electrode and control transference of a charge between the source electrode and the drain electrode, in other words the drive principle is to control an electrical current. FET plays a role of a switching device as it operates the above.
An amorphous thin film silicon or a polycrystalline thin film silicon is traditionally used as the semiconductor of the above described TFT array. Each layer such as the electrode, the semiconductor and the insulting layer of the thin film silicon TFT generally requires a vacuum process and a high-temperature process with more than 300 degrees Celsius. Further, a photolithographic method is used for patterning. This indicates that the TFT is formed by the relatively complicated high-cost process.
Compared with the past, in these days, it has been proposed that a solution dispersion nano-metal particle should be used for an electrode material, an organic semiconductor should be used for a semiconductor and a material which can be solved or dispersed in a solvent such as an organic polymer compound should be used for an insulating material. Further, a great number of a method which uses a coating method such as an ink jet method, a spin coating method or a flexographic printing method has been reported.
Due to this, lowering the temperature, heightening the speed and reducing the cost of the process have been realized.
As a coating material which can be solved or dispersed in a solvent and forms the gate insulating layer, an organic polymer compound, an organic-inorganic hybrid material in which an organo metallic compound is used (referred to patent documents 1-4) and a composite material in which metal oxide particles are mixed (referred to patent document 5 and non-patent documents 1 and 2) have been traditionally studied. In addition, it has been proved that the above mentioned materials can be used as the material of FET gate insulating layer when the gate insulating layer is formed by coating.
The gate insulating layer formed by the organic polymer compound has sufficient insulating properties. However, the gate insulating layer frequently has insufficient FET properties, because the gate insulating layer generally has low relative permittivity of approximately 2.5-3.8. Therefore, the material having high permittivity has been required. Under this circumstance, by adding the metal oxide material which has high permittivity, the organic-inorganic hybrid material and the organic-inorganic composite material are able to have high permittivity which cannot be obtained by only an organic material and hence a use of this kind of materials has been widely studies.
However, there is a problem in a use of the organic-inorganic hybrid material. A thin film is formed by coating the organic-inorganic hybrid material which has the organic metal oxide compound after a hydrolysis process is performed using acid or base. Hence, insulating properties cannot be sufficiently obtained, because the thin film is influenced by acid or base which remains inside of the film. Thus, it led to a new problem such as an occurrence of a high leak current. In addition, when the organic-inorganic composite material is used for FET, since the particles are added to the organic-inorganic composite material, the surface asperity is caused by variation in dispersion of the particles in the film or shape of the particles. Thus, FET properties become poorly-reproducible.
In addition, by adding silicon oxide to the organic-inorganic hybrid material or the organic-inorganic composite material of the related art, stabilization of the material properties are improved. However, in return for the stabilization, the organic-inorganic hybrid material or the organic-inorganic composite material requires a relatively high temperature baking because of its poor reactivity. Further, after the thin film is formed, since the materials have a weak tolerance for a post-processing such as a surface processing for the insulating thin film, FET properties has been made poor because of change in a shape of the surface or chemical properties caused by the hard processing.
Moreover, strong acid or strong base is used for the process for manufacturing TFT, for example, a photoresist development and abrasion which are carried out at the time of patterning by photolithography, and etching the semiconductor, the electrode or the like. Therefore, there is a problem that materials for the electrode, the insulator and the semiconductor formed on a substrate are separated from the substrate by the developing solution or etchant. In particular, when a plastic film is used as a substrate, adhesion between the plastic and the metal or the metal oxide is not sufficient. Moreover, even though adhesion is improved by the surface processing of the substrate such as a UV/ozone treatment or a corona treatment, a bond between the substrate and the metal or metal oxide is chemically degraded because of acid or base. Thus, this problem is remarkable.
The present invention provides an insulating thin film and a formation solution for the insulating thin film. The insulating thin film can be formed by coating the solvent at a low temperature and has high insulating properties and high tolerance for a solvent. In addition, a surface processing can be performed on its surface. Further, the purpose of the present invention is to provide a field-effect transistor having excellent properties, a method for manufacturing thereof and an image display unit, in which the insulating thin film is utilized.    Patent Document 1: JP-A-2005-120371    Patent Document 2: JP-A-2006-070029    Patent Document 3: JP-A-2008-147410    Patent Document 4: JP-A-2008-166764    Patent Document 5: JP-A-2004-055649    Patent Document 6: JP-A-2004-200365    Patent Document 7: JP-A-2001-352068    Patent Document 8: JP-A-2002-28961    Patent Document 9: JP-A-2003-258260    Non-patent Document 1: J. Am. Chem. Soc. 127, 14655 (2005)    Non-patent Document 2: Appl. Phys. Lett. 85, 3295 (2004)